Active scanning beams: 1. Modulating delivery

Transcription

1 Active scanning beams: 1. Modulating delivery Eros Pedroni Paul Scherrer Institute SWITZERLAND Zurzach E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

2 Summary 1. Beam delivery options 2. Scanning technology: experience with Gantry 1 3. Dose precision issues 4. Practical advantages of scanning 5. The organ motion problem link to the next talk new developments of Wednesday 6. Questions E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

3 1. BEAM DELIVERY OPTIONS FOR SCANNING E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

4 1.1 The basic element the proton pencil beam Common to all delivery methods Pencil beams with zero phase space and given initial energy Difference between beam delivery methods? The cumulated phase space at nozzle exit (beam formation) The time structure of the beam and beam delivery sequence Which is relevant for organ motion errors Typical physical scanning beam x= ±3 mm θ= ±10 mrad δp/p = ±0.5% 1 mm sigma 3 mm sigma 10 mm sigma Monte Carlo Scanning Wobbling After integration in the lateral direction the differences in depth disappear E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

5 1.2 Options for beam spreading in the lateral direction Scattering (static) Constant particle fluence (homogeneous dose field) Single scattering (good penumbra but low efficiency) Double scattering (higher efficiency less good penumbra) Contoured 2nd scatter (or double contoured to compensate energy mixing) Field size and depth dependent solutions Uniform scanning (dynamic) Circular (wobbling) - rectangular spiral BEV shapes Conformal scanning (dynamic) Modulated particle fluence (for 3d-conformation and IMPT) Beam motion magnetic Or through patient table motion Performance issues: Precision of the dose shaping Scanning speed > repainting capability > to reduce organ motion sensitivity E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

6 Lateral scanning options Spot scanning Switch beam OFF in between spots PSI Gantry 1 Let beam ON in-between spots GSI raster scanning Continuous scanning Magnet-driven scan Dose shaping by changing the magnet speed (at constant dose rate) Time-driven scan The beam moves with maximum speed the dose is painted by modulating the beam intensity PSI Gantry 2 (feasibility?) Homogeneous energy layer Non-homogeneous energy layer E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

7 1.2 Options of beam formation in depth (Bragg peak) Rotating wheel (very fast ms / cycle) Fast SOBP mixing (quasi-static) Must be designed for each energy - SOBP extent - field size Not of interest in combination with scanning Ridge filter (miniature structure blurred by angular confusion) SOBP energy mix (static) Must be designed for each - energy - SOBP extent - field size Concern angular confusion of the beam - penumbra Range shifter (dynamic 30 ms) - > see PSI Gantry 1 Plates (regular or digital) - moving wedges - water column Range steps or continuous Concern beam spot broadening in the air gap to the patient Variable energy of the beam (dynamic) -> see PSI Gantry 2 Range steps Concerns very steep Bragg peaks at the lowest energies Best in combination with a ridge filter or preabsorber? E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

8 Range shifter -> PSI Gantry 1 Dynamic beam energy -> PSI G2 Variable amount of material in the beam With degrader and beam line Water-equivalent? See future developments talk Material, geometry, density of the plates With zero air gap Pencil beam size invariant with range With non-zero air gap - problematic the Spot broadening due to MCS in the RS E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

9 1.3 Relation between proton energy and fluence for conformal dose shaping Starting example : hat box Uniform proton fluence Fixed SOBP BEAM Collimator Patient Target 100% (80-50%) Fixed range modulation = unnecessary dose delivery 100% (0%) Depth Gray shading = density of proton stops E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

10 Compensator Collimator Compensator Intensity modulation within energy-layer Delivery spot by spot Unnecessary Dose delivery MLC Shape shrinking Uniform Scanning Scanning: Freedom to deliver any pattern OAR OAR Collimator compensator scattering-wobbling Stacking of energy-layers Variable range modulation 3d or 4d-shaped dose conformal scanning E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

11 2. PRACTICAL DEVELOPMENT OF SCANNING THE PSI EXPERIENCE WITH GANTRY 1 E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

12 2.1 PSI Scanning Development started in 1989 With limited resources Compromises due to the limited space of the area Parasitic use of the PSI beam With very long shut-downs E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

13 The decision to build Gantry 1 was taken in 1992 Until May 2008 the only gantry with scanning (1st patient in 1996) Magnetic scanning started before the last bending magnet parallel scanning (but only along one magnetic axis) gantry radius reduced to only 2m Eccentric mounting of the patient table on the gantry front wheel Patient moves away from the floor when treating with beam from below (a drawback) α rotation Upstream scanning eccentric design β rotation φ rotation E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

14 If I could do it again Eccentric compact Gantry 2 Eccentric mounting as with Gantry 1 (R = 2m) But with rotation only on one side (0 180 ) - as with the new Gantry 2 With a counter-rotation False floor underneath the patient table moving with the gantry E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

15 2.1 Scanning on the PSI Gantry 1 Discrete pencil beam scanning Gaussian pencil beam of 3 mm sigma Cartesian scanning (infinite SSD) Step and shoot spot on a 5 mm grid The sequence of the elements of scanning: Time Spot-Dose Monitor + Fast Kicker X Sweeper magnet most often used Y Range shifter 2 nd loop Z Patient table slowest loop Weak point: transverse scanning by moving patient table Slow motion ( no repainting possible) We can treat only non moving targets Head, spinal chord and low pelvis E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

16 Dimensional considerations for scanning Reference size of 1 liter (very often much less, our max value 4 liters) Assumed beam size: 3 mm sigma (see next section) Derived grid size: 5 mm (21 spot lateral - 23 in depth) 21 x 21 x 23 ~ spots/liter 21 x 21 x 10 cm = 44 m path length Assumed treatment time: 1.5 minute beam-on time 10 ms/spot in average Due to the non-uniform spot weights (for a uniform SOBP) Most distal spots: 60 ms Most proximal: 3 ms (with beam spot weight optimization: we accept spots >= 0.5 ms) Treatment roughly proportional to - VOLUME (the moving time) and to BEV SURFACE (beam ON time of the high weighted distal Bragg peaks) Required intensity: 0.2 na proton current beam ON time of 1 Min to deliver 1 Gy in1 Liter E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

17 Beam monitoring Transmission ionization chambers (M1 and M2) proton flux 5 (10) mm in air 2kV voltage response time < 100 μs > precision of controlling the dose 1% of the mean spot time (switching time of the kicker 50 μs) Delay of the current measurement and of the kicker subtracted in advance from preset -> 0.2% of mean spot time Strip-monitor chamber 4 mm strips Measure position and width of the beam after the delivery of each spot Position resolution: < 0.5 mm Charge collection time ~ 0.8 ms Wait 1 ms before reading scalers at the end of the spot E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

18 The reasons to switch off the beam in-between spots Avoid dose uncertainties when stepping with beam ON to the next spot (transients) Dose errors (sweeper power supplies delay and non linearities) Errors in checking the beam position with the strip monitor (time resolution of 0.8 ms at 1 cm/ms => 8 mm error) Poor quality of the beam until 2006 (before COMET) Beam splitting ( 0.5% intensity) from the 2 ma beam of the PSI ring cyclotron and degrading it from 590 MeV down to MeV After any repair -> bad vacuum - > intensity spikes Check monitor units precisely at the end of spot Conservative strategy Perform all calculations at the end of a static spot Start next spot only when previous delivery shown to be correct -> overall dead time of 5 ms in-between spots Best approach to control the dose with repainting Beam IC Sweeper Hall probe E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

19 Scanning motion devices Sweeper magnets 30 ms for a full sweep of 20 cm For a 5 mm step Time to move the beam and to stabilize: 3 ms < time to check previous spot: 5 ms (Gantry 2 2ms) Range shifter 40 plates (80 pneumatic valves) 4.5 mm thick each + one half plate Water equivalent arrangement Dead time 50 ms (30 ms for motion) Patient table Moves in steps of 5 mm 1-2 seconds dead time per step Acceleration and decelaration Smooth motion for patient comfort E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

20 The price to pay: the overall scanning dead time Gantry 1 Beam off after delivery of each spot Sweeper dead time x 5 ms = 50 s Range shifter 21x21x 50 ms= 22 s Patient table 21 x 1s = 21 s 1.5 minutes beam off vs. 1.5 minute beam on Duty factor of our present discrete scanning low 50% Precision of dose delivery very good Dose reproducibility of ~ 0.2% Future: exploring more efficient solutions with Gantry 2 Painting of lines instead of spots (check of dose delivery at the end of a line) -> see future developments presentation E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

21 The required precision of placing the spots Beam delivery Small systematic errors in the sweeper calibration Relative beam position precision of± 0.2 mm from one spot to next Target motion during beam delivery (patient) Statistical error Gets reduced by sqrt( N*m*k) N= number of fraction m= number of fields Organ motion error is a function of ratio k= repainting number Motion size / pencil beam size Practical limit for Gantry 1 Motions < ± 1-2 mm (including full fractionation N=30) E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

22 The precision of setting the beam (beam tunes) The reproducibility of the beam tunes Automatic set-up of the beam energy without retuning the beam during treatments (cyclic ramping of the gantry magnets) The beam appears at the correct position within 1.5 mm Position correction at the end of the first spot Correction allowed if within ±1.5 mm All further spots Position deviation within ± 0.5 mm of the expected value Interlock if deviation > 1.5 mm 1 mm Beam position error On-line and off-line analysis of delivered spots Analysis of logged data U sweeper position E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

23 3. DOSE SHAPING PRECISION OF SCANNING E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

24 3.1 Distal fall-off Function of energy (linear with range) Range straggling (σr/r ~1%) unavoidable Momentum band avoidable Very sharp Bragg peaks Reason to use a pre-absorber at low energies (for variable beam energy) Energy dependence Momentum band dependence Desirable Δp/p At highest energy ~0.2% FWHM At lowest energy ~2% FWHM cm E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

25 Distal fall-off Scanning with variable energy In principle superior to scattering No material in the beam (no compensator no range shifter) Minimalst energy for the required range (physical limit) Less range straggling Sharper distal fall-off In practice the advantage compared to scattering is probably very marginal E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

26 3.2 Lateral fall-off Governed by the spot size within the patient (dose spot) at the Bragg peak (the major effect) Derived from The beam broadening due to the Multiple Coulomb Scattering in the patient Given by the physics unavoidable In doesn't help to use a beam size which is much smaller than this effect The beam size in air (size of the beam at the exit of the nozzle) Given by the beam delivery system by design Accelerator - beam line - and the material in the nozzle The smaller the dose spot -> the sharper the lateral fall-off Sigma lateral fall-off ~=~ sigma dose spot E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

27 Optimization of the beam size (spot size) Beam pipe Nozzle Beam in vacuum Provide a beam source with small phase space Beam in air Bring the vacuum very close to the patient Beam monitors (material in the nozzle) Place material close to the nozzle exit Reduce amount of material (of low atomic number) Reduce air gap to the patient!!!! Range shifter or pre-absorber Reduce air gap to the patient!!!! Patient MCS You can do nothing about this Size = θ x Distance θ = F(E) x sqrt (Material / S) E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

28 The air gap problem when using a range shifter Beam blow-up due to MCS in the range shifter in front of the patient The reason with Gantry 2 to go for variable beam energy FWHM (cm) 119 MeV MeV 10 0 FWHM (cm) 214 MeV FWHM (cm) Gap(cm) Gap(cm) Gap(cm) Strategy of positioning beam modifiers in the beam Either very close to the patient Small air gap Or very far Loss of intensity But not in-between The worse you can do Similar problem with scattering air gap to compensator lateral penumbra E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

29 Patient MCS MCS in the patient PMCS Depends linearly with range Rule of thumb 1 cm FWHM at 20 cm 1.5 cm FWHM at 30 cm Choice of the beam size (BS) in air Sum in quadrature of the beam size in air (BS) and the MCS in the patient (Pmcs) if we choose BS < 50% Pmcs we obtain sqrt( Pmcs^2 + BS^2) < 1.12 Pmcs A too small beam size -> needs smaller scan grid > more spots -> more dead time and is more sensitive to organ motion errors E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

30 Edge enhancement capability of scanning Delivery of separated spots Variable choice of intensity The dose lateral fall can be made to be similar to the fall-off of the original beam Gaussian Uniform fluence of spots The case of collimation Gaussian folded with step function = error-function Max difference Factor 1.7 (1.4) Scanning with optimization can produce a sharper lateral falloff as compared to scattering E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

31 Beam width at the Bragg peak as a function of the range Assumed phase space: 6 mm FWHM constant in vacuum at all energies Relevant for organ motion errors practical limit - beam line Nozzle material: as for Gantry 1 practical limit - monitoring MCS in the patient enhanced by factor a 1.4 in favor of scanning Collimation improves precision only at ranges below 11 cm Idealized scattering (zero phase space) Realistic scanning Factor 1.4 higher due to the errorfunction Scanning with collimation better Scanning alone better E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

32 3.2 Lateral fall-off Scanning with variable energy (with or without collimator) In principle superior to scattering No material in the beam (no compensator no range shifter) No MCS propagating in the air gap sharp beam Less angular confusion sharp shadow when using collimators E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

33 Options for collimated scanning Scanned field Sensitive structure SCANNING ONLY APERTURE CONTOURED APERTURE LESS WEIGTH BETTER VISIBILITY FOR BEV X-RAYS OAR- SHIELDING BLOCK E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

34 Possible scanning patterns Spot scanning (G1) Raster scanning (GSI) Criteria Precision (edge of the field) Duty factor (beam-off time) Topological scan (repainting capability) Spot scanning++ (G2) Line scanning (G2) IM Contoured scanning (G2) IM E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

35 4. ADVANTAGES OF SCANNING E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

36 All done by software with minimal equipment No need to use individualized hardware Avoid fabrication and mounting of patient specific equipment in the nozzle Apply dose fields sequences in one go without personnel entering in the treatment room To reduce treatment time All fields of IMPT are delivered in the same fraction Most efficient use of the beam All used protons reach the target Minimal neutron background (for the patient) by default Less activation of equipment (for the personnel) by default Flexibility to treat from small to very large fields without changing equipment All done by the beam Sharper penumbra than scattering (scanning alone or collimated) (Gantry 2) Less material before the patient (no compensator) E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

37 Variable modulation of the range For avoiding unnecessary 100% dose on the healthy tissues Especially relevant for large tumors E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

38 4D (dose-modulated fields) Dose tailored geometrically in 3d Dose shaping within the target Used for Intensity modulated therapy IMPT Biological targeting Competition with IMRT E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

39 Other possible scanning related advantages Gantry design with upstream scanning Reduce the radius of the gantry No additional radial distance for the spreading the beam Parallelism of scanning source at infinite distance Simplify dosimetry treatment planning field patching - collimation and compensation Capability to simulate scattering (repainted BEV box scans) Scanning can simulate and improve scattering (variable range modulation) The opposite is probably not true Provide backward compatibility with the more established techniques using one and the same nozzle (see future developments -> Gantry 2) The only drawback compared to scattering The SENSITIVITY TO ORGAN MOTION ERRORS A problem common to all dynamic beam delivery methods including IMRT E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

40 5. THE ORGAN MOTION PROBLEM E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

41 5.1 An unsolved problem: organ motion errors of scanning Disturbance of the lateral dose fall-off (same problem for scattering and scanning) Add safety margins or Reduce with Gating or Tracking Disturbance of the dose homogeneity Scattering highly repainted - insensitive Single painted scanning - very sensitive Repainted scanning - acceptable Alone for medium motion With Gating or Tracking for large motion The experience of treating moving targets with scanning is still inexistent WE HAVE TO LEARN HOW TO DO THAT Prospective solutions see next talk on future developments with Gantry 2 E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School

42 A very exciting field Based on a beautiful idea The next step scanning speed THANK YOU E. Pedroni Center for Proton Radiation Therapy - Paul Scherrer Institute - Proton Therapy Winter School